Are you using this for the two dates?
  Cells(currentrow, 21).Value = CDate(Start1)

  Cells(currentrow, 26).Value = CDate(Start2)
PS Why are you using variables for the values from the textboxes? Can't you just write directly from the textboxes to the cells?
  Cells(currentrow, 21).Value = CDate(txtStartdat1.Text)

  Cells(currentrow, 26).Value = CDate(txtStartdat2.Text)